1. Field of the Invention
This invention relates to the field of radio frequency transmission. More particularly, this invention relates to the field of a self-tuning digital output filter for use in a delta-sigma radio frequency transmitter.
2. Description of Related Art
A continuing challenge in the transmission of wireless signals is the process of modulating a carrier signal, i.e., varying the amplitude and/or phase (frequency) in such a way as to convey information onto that carrier, in a mathematically ideal way without producing noise or distortion. Traditional systems have used either analog circuitry exclusively, or have used digital circuitry to generate a baseband or intermediate frequency (IF) waveform followed by conventional analog circuitry to translate the signal up to the required carrier frequency. In either case, the use of conventional analog circuitry has resulted in the generation of spurious signals due to the inability of analog circuitry to accomplish mathematically ideal operations.
U.S. Pat. No. 6,748,025, assigned to the assignee of the present invention, discloses the use of commutation in combination with delta-sigma conversion in the architecture of an A/D converter based linear receiver.
U.S. patent application Ser. No. 10/998,212, published as publication number US 2005/TBD and entitled Direction Conversion Delta-Sigma Transmitter, describes a transmitter that replaces analog superheterodyning with commutation as the basis for upconverting a baseband information signal to a radio frequency signal. The transmitter structure disclosed in that application uses a delta-sigma modulator followed by a commutator to upconvert the signal directly from baseband to the desired carrier frequency, followed by a discrete time analog filter to suppress the delta-sigma quantization noise outside of the band ωcarrier±ωbaseband that is introduced by the commutation. FIG. 1 is a representative plot of the spectral content of the output of the delta-sigma transmitter transmitting at a carrier frequency of 2 GHz, before filtering by the digital filter. As described in the above-referenced patent application, after filtering, the output spectrum of the transmitter is a spectrum that is “clean” (having a low noise floor) in the neighborhood of the carrier frequency with the quantization noise floor rising with increasing frequency above the carrier frequency and with decreasing frequency below the carrier frequency.
An important advantage of using a delta-sigma transmitter is that the carrier frequency can be changed over a wide range merely by changing the clock frequency, which can be accomplished using software in combination with simple hardware. FIG. 1 shows the result of commutating a baseband signal with a (−1)n waveform at a clock rate of 4 GHz, for illustration's sake. The spectrum is now centered around 2 GHz with a low noise floor in the neighborhood of the carrier frequency. Although obscured by the resolution of the plot, there are two separate sidebands, which is the expected result of double sideband suppressed-carrier modulation.
As with all delta-sigma converter outputs, a separate filter is required to suppress the quantization noise floor so that only the intended output waveform remains. More precisely, depending on the exact application and the specifications for it, a separate filter is required to substantially suppress the quantization noise to a level that is acceptably low for that application. For the transmitter, a bandpass filter centered at the carrier frequency is preferred. This filter does not need to be highly selective since the rate at which the noise floor rises is very gradual for high clock rate converters. However, this bandpass filter must be tunable since a software radio is required to be-frequency agile. While practical methods for electronic tuning, such as varactors in conjunction with switched values of inductors, may be used to implement a frequency agile bandpass filter, it would be far more useful to implement a bandpass filter that is both highly selective and has a tunable center frequency.